Display screen or portion thereof with a transitional graphical user interface



FIG. 1 is an enlarged front view of a first image in a sequence according to a first embodiment of my design for a display screen or portion thereof with a transitional graphical user interface; and

FIG. 2 is an enlarged front view of a second image in the sequence thereof.

FIG. 3 is a front view of a first image in a sequence according to a second embodiment of my design for a display screen or portion thereof with a transitional graphical user interface; and,

FIG. 4 is a front view of a second image in the sequence thereof.

The appearance of the transitional image transitions between the images shown in FIGS. 1-2 and 3-4. The process or period in which one image transitions to another forms no part of the claimed design.

The long dash broken lines show an electronic device for the purpose of illustrating environmental structure and form no part of the claimed design. The dash-dot-dash broken lines show portions of the graphical user interface and form no part of the claimed design. The dash-dash broken lines show a display screen or portion thereof and form no part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a transitional graphical user interface, as shown and described. 